You want Moisture Barrier Bags (MBB). It is possible you are calling these bags, standard anti-static bags. If you can see anything through the bag, they are not what you want to use for vacuum sealing. In any case make sure you get MBBs.

Moisture barrier bags are polymer coated foil so are nearly gas impermeable. There's always some coating left at the weld (or it wouldn't stick) so they're not completely sealed but that's as good as it gets.

Usually people use the vacuum to remove air and then replace with nitrogen to eliminate oxidation and a medium for bacterial growth. Why anyone would need to vacuum pack a pcb is beyond me, though.

"I read a study a while back by IBM that showed too strong a vacuum would cause too much moisture to enter the bags."

That's what the nitrogen is for. If you pull a vacuum w/o backfilling with something dry, like nitrogen, pretty soon the vacuum overcomes the bag permeability (and pump, vacuum chamber, etc.) and outside air ends up getting in. Granted, if you're pulling that kind of vacuum you're probably perforating your bag on components anyway so it's kind of pointless.

Playing with vacuum is fun. You'd be amazed the trouble you can get into wth a system capable of pulling down to

For a day or a week probably. Possibly a month. But I"m sure not for a year or such. There would be too many variables to know exactly how long it would be before the desicant became saturated. And it wouldn't meet the Jedec standard.

"Usually people use the vacuum to remove air and then replace with nitrogen to eliminate oxidation and a medium for bacterial growth. Why anyone would need to vacuum pack a pcb is beyond me, though. "

We might get a batch of 400 pcbs in but the kit might only clear for 200 - these boards are either left on the shelf or they need to be baked and then vacuum packed for storage ( am I putting extra work on us for doing this ?)

We have undoubtedly seen an increase in issues with PCBs since the transition to Leadfree and on one particular batch which we seen delamination - baking these boards got rid of the problem. So what I am trying to do now is get stores to bake and vauum pack any boards that are causing us problems.